The present invention generally relates to a temperature measuring device, and particularly to a temperature measuring device which employs a temperature-sensitive resistor such as a thermistor or a posistor. Further, the present invention relates to a thermal head device having such a temperature measuring device. The present invention is suitable for adjusting electrical energy supplied to thermal elements arranged in a thermal head on the basis of a temperature variation thereof in order to obtain uniform printing characteristics.
Currently, thermal printers are widely used. A thermal printer employs a thermal head, which includes a number of thermal elements. In order to obtain uniform printing characteristics, it is required to adjust the power supplied to thermal elements arranged in a thermal head, depending on a temperature variation thereof. For this requirement, conventionally, a thermistor is mounted on the thermal head. A thermistor changes its resistance in response to a variation in temperature. Power supplied to thermal elements is controlled by adjusting the pulse width of a pulse supplied thereto based on variations in temperature detected by the thermistor.
Japanese Patent Publication No. 61-28516 discloses a temperature measuring device using a thermistor. The disclosed device directly measures a resistance of the thermistor by a resistor and a comparator. The resistance value of the thermistor is converted into a voltage signal by the resistor. The comparator compares the voltage signal with a plurality of reference voltages. The comparison results indicate the resistance value of the thermistor. Alternatively, the resistance value of the thermistor may be obtained by extracting a voltage signal by using an analog-to-digital converter.
Japanese Laid-Open Patent Application No. 60-13569 discloses a temperature measuring device in which the resistance value of a thermistor is measured by converting the resistance into a frequency signal by a generator including a non-stable multivibrator. The pulse width to be supplied to thermal elements is adjusted according to the measured frequency.
As is well known, it is very difficult to manufacture thermal heads each having a plurality of thermal elements and each exhibiting almost the same value of composite resistance of the thermal elements. That is, the composite resistance value of thermal elements is different for different thermal heads. Therefore, the average composite resistance value of the thermal elements is measured for every thermal head during a manufacturing step.
Conventionally, dispersion of the resistance values of thermal elements is taken into account as follows. The average composite resistance value of the thermal elements is measured for every thermal head during a manufacturing step. The measured average resistance value obtained for each thermal head is written on a suitable portion thereof. Alternatively, the optimal pulse width for to the obtained resistance value is written. At the time of assembling a thermal printer, the optimal pulse width obtained for every thermal head is registered in a memory provided in a controller for controlling the thermal printer. In operation, when a variation in temperature of the thermal head is detected, and the optimal pulse width to be set at that time is determined from the stored pulse width and the measured temperature variation.
Japanese Laid-Open Patent Application No. 61-29558 proposes a temperature measuring device, which takes account of the dispersion of the resistance values of thermal elements. The proposed device has a head resistance identification code generator. A predetermined number of ranges of the average resistance values is provided so as to cover the possible average value of resistance of thermal elements. The generator is designed to output a identification code indicative of one of these ranges. Then, the generator is adjusted so as to output the identification code related to the average value of resistance over all thermal elements provided in the thermal head of concern. For this purpose, the generator includes switches or jumper wires each provided for the respective ranges. The switches or jumpers are connected to a resistor network provided outside the thermal head. The identification code is used for adjusting the pulse width applied to the thermal elements in addition to the detected temperature variation.
However, the temperature measuring device disclosed in Japanese Patent Publication No. 61-28516 has a disadvantage in that the device is complex. The device disclosed in Japanese Laid-Open Patent Application No. 60-13569 has a disadvantage in that the measurement of frequency change requires a large number of structural elements. Further, the aforementioned setting of the optimal pulse width is very troublesome because when a thermal head provided in a thermal printer is replaced with new one, it is required to rewrite the optimal pulse width stored in the memory. The Japanese Laid-Open Patent Publication No. 61-29558 presents the following disadvantages. That is, when the average value of resistance of the thermal elements is over a wide range, it is necessary to provide a number of switches or jumper wires. This makes the device complex. Additionally, since the device uses the switches or jumper wires, it is impossible to form the entire temperature measuring device on an integrated circuit chip.